At the time, Singh — an early supporter of de Blasio’s bid for the mayoralty — was embroiled in a dispute with the city over the lease that let him run Water’s Edge, a since-closed restaurant on a barge next to a publicly-owned pier in Long Island City.

Several DCAS staffers were repeatedly told by de Blasio’s aides to “resolve this matter” because Singh was “a friend of the mayor,” according to the DOI memo.

DCAS staffers described Cumberbatch’s personal involvement — which included meeting Singh and top de Blasio aide Emma Wolf in City Hall in 2015 — as unprecedented for such a minor dispute, the memo says.

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In March, acting Manhattan US Attorney Joon Kim announced that neither de Blasio nor anyone who had acted “on his behalf” would be charged as a result of the federal investigation.

Singh, however, was busted in 2015 for several unrelated schemes, including an alleged $950,000 fraud involving federal Hurricane Sandy disaster-relief funds tied to Water’s Edge, and is currently awaiting trial.

In addition to the revelations regarding DCAS and Cumberbatch — who transferred to the city’s municipal hospital system last year — a related DOI letter revealed that de Blasio and four top aides “regularly” used personal email accounts for official business.

The DOI memo says the mayor and his aides, including Wolfe and Deputy Mayor Anthony Shorrris failed to “consistently” forward the emails to their official accounts, “as they should have in accordance with their internal Record Retention and FOIL instructions.”

De Blasio spokesman Eric Phillips said City Hall “frequently” reminds city officials “to use government email for government work….and we’ll continue to do that.”

He also said about the de Blasio’s call to Cumberbatch: “Whether or not he called her or she called him, they’ve spoken on the phone other times, and he’s certainly talked to her in person on any number of occasions.”

“This administration makes decisions based on the facts and nothing else,” Phillips added.